This invention relates to a significant improvement in glass or glass-ceramic to metal seals and processes for their formulation.
Glass or glass-ceramic to metal seals are generally fabricated in a two-step process. Firstly, the glass component and the metal component are preliminarily bonded to one another using molten glass followed by cooling or a glass preform followed by heat or RF energy. Thereafter, the initially formed preliminary seal is subjected to a time-temperature "seal cycle" during which the final seal is made. In addition, as is well known, in appropriate systems, the glass to metal seal is transformed into a glass-ceramic to metal seal comprising various crystalline forms (quartz, cristobalite, etc.), the precise form of the crystallized phases depending on the composition and conditions employed. This time/temperature cycle is carried out under ambient pressure. See, e.g., McMillan et al, Glass Technology, Vol. 7, No. 4, Aug., 1966, pp. 121-127, whose entire disclosure is incorporated by reference herein. Alternatively, the foregoing process can be carried out in a single time/temperature cycle.
The quality of the finished seal is dependent on a multitude of factors and it is normally characterized in terms of burst strength, hermeticity, porosity, and the extent of the reaction between the glass ceramic and the metal component as a result of the processing wherein the glass or glass-ceramic finally flows onto and seals with the metal component parts.
Many disclosures describe the general method outlined above and also other related methods. See, e.g., U.S. Pat. No. 2,876,596 which uses a hot press to apply pressure (up to 7,000 psi) while heating to a temperature just above the strain point of the glass but well below the melting point of the glass; U.S. Pat. No. 4,159,075 which also uses similarly low pressures (e.g., up to 100 N/cm.sup.2) to seal glass to a metal or other bonding material without melting of the glass; McMillan, supra, which mentions the use of pressure to regulate the flow of the glass during the first melting step; U.S. Pat. No. 3,220,815 which discusses the pre-treatment of metal substrates prior to the sealing operation; U.S. Pat. No. 2,219,573 which also employs classical unidirectional pressure to increase the flow of glass during the metal contacting first step; and U.S. Pat No. 2,392,314 which relates to various preferred glass compositions for use in the prior art process.
The quality achievable using state-of-the-art techniques is still inadequate for many purposes, especially the more demanding ones.